xref: /src/sys/lib/libkern/arch/sparc64/random.S

/*	$NetBSD: random.S,v 1.1 1998/06/20 05:18:14 eeh Exp $	*/

/*
 * Copyright (c) 1990,1993 The Regents of the University of California.
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that: (1) source code distributions
 * retain the above copyright notice and this paragraph in its entirety, (2)
 * distributions including binary code include the above copyright notice and
 * this paragraph in its entirety in the documentation or other materials
 * provided with the distribution, and (3) all advertising materials mentioning
 * features or use of this software display the following acknowledgement:
 * ``This product includes software developed by the University of California,
 * Lawrence Berkeley Laboratory and its contributors.'' Neither the name of
 * the University nor the names of its contributors may be used to endorse
 * or promote products derived from this software without specific prior
 * written permission.
 * THIS SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS OR IMPLIED
 * WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
 *
 * Here is a very good random number generator.  This implementation is
 * based on ``Two Fast Implementations of the "Minimal Standard" Random
 * Number Generator'', David G. Carta, Communications of the ACM, Jan 1990,
 * Vol 33 No 1.  Do NOT modify this code unless you have a very thorough
 * understanding of the algorithm.  It's trickier than you think.  If
 * you do change it, make sure that its 10,000'th invocation returns
 * 1043618065.
 *
 * Here is easier-to-decipher pseudocode:
 *
 * p = (16807*seed)<30:0>	# e.g., the low 31 bits of the product
 * q = (16807*seed)<62:31>	# e.g., the high 31 bits starting at bit 32
 * if (p + q < 2^31)
 * 	seed = p + q
 * else
 *	seed = ((p + q) & (2^31 - 1)) + 1
 * return (seed);
 *
 * The result is in (0,2^31), e.g., it's always positive.
 */
#include <machine/asm.h>

	.data
randseed:
	.long	1
	.text
ENTRY(random)
	sethi	%hi(16807), %o1
	wr	%o1, %lo(16807), %y
	 sethi	%hi(randseed), %g1
	 ld	[%g1 + %lo(randseed)], %o0
	 andcc	%g0, 0, %o2
	mulscc  %o2, %o0, %o2
	mulscc  %o2, %o0, %o2
	mulscc  %o2, %o0, %o2
	mulscc  %o2, %o0, %o2
	mulscc  %o2, %o0, %o2
	mulscc  %o2, %o0, %o2
	mulscc  %o2, %o0, %o2
	mulscc  %o2, %o0, %o2
	mulscc  %o2, %o0, %o2
	mulscc  %o2, %o0, %o2
	mulscc  %o2, %o0, %o2
	mulscc  %o2, %o0, %o2
	mulscc  %o2, %o0, %o2
	mulscc  %o2, %o0, %o2
	mulscc  %o2, %o0, %o2
	mulscc  %o2, %g0, %o2
	rd	%y, %o3
	srl	%o2, 16, %o1
	set	0xffff, %o4
	and	%o4, %o2, %o0
	sll	%o0, 15, %o0
	srl	%o3, 17, %o3
	or	%o3, %o0, %o0
	addcc	%o0, %o1, %o0
	bneg	1f
	 sethi	%hi(0x7fffffff), %o1
	retl
	 st	%o0, [%g1 + %lo(randseed)]
1:
	or	%o1, %lo(0x7fffffff), %o1
	add	%o0, 1, %o0
	and	%o1, %o0, %o0
	retl
	 st	%o0, [%g1 + %lo(randseed)]